Thermal time lag fuse



Aug. 14,1951

G. A. BERTHEL 2,563,812

THERMAL TIME LAG FUSE Filed Aug. 50, 1948 W Q W Patented Aug. 14, 1951THERMAL TIME LAG FUSE .Gust A. Barthel, Chicago, Ill., assignor toJefferson Electric Company, Bellwood, 111., a corporation of Illinoisv.Application AugustBO, 1948, Serial No..46,7.39

12 Claims.

This invention relates to an improved thermal time lag fuse of the plugtype.

'It' is an object of my invention to provide an improved delayed actionplug fuse, which can'be used in combination-with circuits, such as motorcircuits, where'it'is desiredto accommodate small overloads for'a-considerable period of time, or a moderate-overload fora short periodof time, as well :as to provide the necessary instantaneous blowingunder short circuit conditions.

The ordinar plug fuse'is provied with a fuse link, the dimensions ofwhich are such as to provide the necessaryprotection for small overloads. When .such a fuse is used, in motor circuits, 'for instance, itisfound that the fuse will often blow under .the moderate overloadocoasioned by the :current drawn by the'motor under starting conditions.It is sometimes the practice to substitute aifuse having ahigher ratingin order to :accommodate these moderate overloads. However, this is ahighly undesirable practice, and is dangerous because it affords noprotection whatsoever for isma'llsoverloads.

Various means have :beenproposed to provide a proper time lagcharacteristic which will overcomethese difficulties. Howevenmost ofthe'time lag constructions, by virtue of their dimensions, are notsuited for use in a plug fuse, in'which the space is necessarilylimited. The present improvement contemplates the provision of improvedcircuit breaking means in combination with a fuse link of somewhatgreater current carrying capacity than that which would ordinarily beused .in a. fuse of the same rating. The two are connected in seriescircuit so that on smalland moderate overloads the circuit breakingmeans opens before the fuse .link, and on heavyroverloads and shortcircuits, the fuse link blows-first.

.The .circuit breaking means comprises the combination .of a resilientelement and .a. releasable connection for holding the resilient elementin adeformed and stressed condition.

The time delay on small and moderate overloads is occasioned'by theheatabsorbing capacity of the parts. Where the parts are small, it isdifficult .to provide small rating fuses of uniform characteristicssince the quantity of solder or fusible metal used in effecting theconnection is subject to considerable variation. In other words, insoldering two parts together, it is difficult to apply the same amountof solder to each assembly.

'It is another object of my invention to provide an improved fuseconstruction in which the amount of solder can be readily limited toapredetermined amount. In one form of my invention the heat absorbingcapacity is provided primarily by a separate lagging element and .theamount of solder requiredis sufficient only'to intimately associatetheparts in heat transmitting relationship. In another form of myinvention, wh re less lagging is required as in fuses of lower ratings,the solder itself provides the required'heat absorbing capacity.

In both instancesthe heat absorbing means is of comparatively smallmass, thus dispensing with the use of a separate heating coil andproviding a construction that can be readily supported by the resilientelement alone. The construction of the parts is 'suchthat the mass ofthe heat absorbing material can be held within comparatively closetolerances.

Where a separate lagging element is used, it is preferably die cut fromstrip stock of uniform thickness. Thus the characteristics of a givenproduction run offuscs willbe identical to each other. Furthermore, byusing a releasable con= nection of this type, fuses of different ratingscan be provided merely by "changing the size and shape of the die cutheat absorbing element, and by changing the thickness of the stripmaterial from which this element and certain other elements are formed.

Where the separate lagging element is'not used, the end portion of theresilient element may be shaped so as to hold a predetermined quantityof solder.

In both instances, the end :portion of the resilient element is slottedand the resilient element is maintained in its deformed and stressedcondition by a fuse link which also serves to generate part of therequired heat. The end portion of the fuse link passes through the slotin the resilient element, and the parts are secured together in such amanner that the heat absorbing material which is supported by theresilient element is in intimate heat transmitting relationship withboth the resilient strip and the fuse link. Thus, localized heating ofany partof the solder is avoided which would cause variation inelectrical characteristics.

A still further object of my .invention is toprovide a delayedactionrplug fuse :of comparatively few metal parts, all of which can be.manufac tured by a punching process thereby providing enhanceduniformity of mechanical and electrical characteristics which results inlow unit cost.

Other objects, features and advantages will become apparent as thedescription proceeds.

With reference now to the drawings in which like numerals designate like'parts Fig. 1 is a central sectional elevation of a preferred embodimentof my invention;

Fig. 2 is a plan View taken along line 2-2 of Fig. 1 but with the topremoved;

Fi 3 is an elevation of the body member of the fuse before the currentcarrying elements have been assembled therewith, this elevation beingtaken-from right-hand end of the body member as it appears in Fig. 1

Fig. 4 is an elevation of the body member of the fuse taken from theleft-hand end as the body member is viewed in Fig. 1, and also showingcertain of the current carrying parts;

Fig. 5 is an elevation of a fuse link, showing the various portionsthereof before the same has been bent so as to fit over the other partsof the fuse;

Fig. 6 is a perspective view of the fuse link after it has been bent;

Fig. '7 is an enlarged exploded perspective view of the releasableconnection;

Fig. 8 is a view similar to Fig. '7 but showing the parts in assembledrelationship;

Fig. 9 is a view similar to Fig. l, but showing a modified form of myinvention;

Fig. 10 is an enlarged sectional view showing the resilient member andthe fuse link in assembled relationship, and

Fig. 11 is a plan view of the end portion of the resilient member priorto assembly.

As shown in Fig. 1, the fuse plug comprises a body member ID, and a topH. The body member may be formed of any suitable insulating material,such as Bakelite, a cold mold composition, or porcelain, and the topmember I l is preferably formed from glass, thereby permittinginspection of the condition of the fuse. A screw shell I2 is providedexteriorly of the body member ID, and overlaps the lower portion of theglass top II, the lower part of the glass top being provided withcooperating screw threads l3 which interlock with the threads IE onscrew shell l2.

A center contact I 5 is provided in the body member. The currentcarrying parts of the fuse which establish electrical contact betweenthe center contact 15 and the screw shell l2 comprise a resilient member15, the lower end of which is suitably anchored to the center contact, afuse link l1, one end of which engages the shell I2, and solder I8 whichsecures the fuse link and I the resilient member to each other in such amanner that the resilient member is maintained in a stressed condition.When the temperature of the parts, due to overload conditions, increasesand approaches the melting point of the solder It, the resilient member[5 will be released, and thereby open the circuit. The solder ispreferably a bismuth solder having a melting point of substantially 280F., although other low melting point metals may also be used. Theresilient member I 6 is made of a material which is capable ofmaintaining its resilience even though it is subjected to considerableheating. As an example of suitable materials, I have found that a heattreated copper beryllium alloy is satisfactory. The fuse link I! may bepunched or die-cut from strip copper of a thickness ranging from .001 to.005 inch, depending upon the rating of the fuse.

The time delay characteristics of the fuse are produced bythe use of aheat absorbing lagging element which is in the form of a copper slughaving a slot 2| formed therein, as shown in Fig. 7. The slot 2|registers with a slot 22 formed in resilient member I6 and a tongueportion 23 of the fuse link projects through the slots 2! and 4 22 andthe parts are secured together mechanically and electrically and in heattransmitting relationship by means of the solder I8.

The fuse link is shown in Figs. 5 and 6, and in addition to the tongueportion 23, it is shaped to provide a pair 'of shoulders 24 which abutagainst the resilient member I 6 and limit the penetration of the tongueportion into the slots 2| and 22. The fuse link also includes a notchedportion 25 which is adapted to blow under short circuit conditions.Wings 26 are provided which extend laterally along the upper edge of thebody member H]. In Fig. 3, the areas 26a are indicated by dotted linesand constitute the areas of the body member which are contacted by thewings 26. The tail portion 21 of the fuse link can be divided into anupper portion 28 which rests in a recess 30 formed in the cylindricalsurface of the body member ID and int a bent-up portion 29 which isadapted to be confined between the skirt portion of the glass top H andthe screw shell I 2 in such a manner that a soldered electrical contact48 is provided between the fuse link and the screw shell. A notch 3| isprovided in the upper edge of the body member Ill to receive thatportion of the fuse link which i immediately above the wings 26. Thisnotch and the recess 30 aid in positioning the fuse link with respect tothe body member and prevent a shifting of position as the parts areassembled.

Since the fuse link is formed from exceedingly thin stock, it is quiteflexible and difiiculty is experienced in maintaining the correctspacing between the upper end of the resilent member l6 and the oppositeportion of the body member.- For this reason, the wings 26 are providedwhich determine the position of the bend 36, shown in Fig. 6. Thus, thespacing between the bend 36 and the shoulders 24 is predetermined by theshapeof the fuse link, and hence a uniform tension can be maintained onthe resilient member l6.

As shown in Fig. 3, a notch 32 is provided in flange 33 of the bodymember I 9 so that the portion 28 of the fuse link can be maintainedbelow the upper surface of the flange 33. Thus, the body member I!) andthe glass top H can be maintained with close tolerances since there areno parts projecting outwardly from the cylindrical surface of the bodymember.

As shown in Figs. '7 and 8, wings 34 are provided at the upper portionof resilient member l6, and these wings embrace the side edges of thelagging element 2! The solder [8 which secures the fuse link to theresilient member is also caused to flow in between the resilient memberand the lagging element, and in particularly between the wings 34 andthe side edges of the lagging element so that the parts may be securedtogether in good heat transmitting relationship. When the slot 22 isformed in resilient member 16, the material is pushed outwardly to forma supporting projection 35 which projects into the slot 2| of thelagging element and serves to hold the parts in position while thesoldering is effected.

As shown in Fig. l, the end 31 of the resilient member I6 is bent overand is received within a well formed in the center contact [5, and issecured into the well by means of solder 39. Thus electrical contact isprovided, and the lower end of the resilient member is anchored in orderto permit the same to be maintained in stressed condition by the fuselink and releasable consect o M g In operation a short circuit willcause the notched portion 25 of the fuse link to blow. On small ormoderate overloads, however, the heat developed in the resilient memberIt and the fuse link El and in particular, in the notched portion of thelatter will be conducted to the lagging element 2% and will serve tomaintain the solder is below its melting point for a predeterminedperiod of time. When this overload condition continues beyond thispredetermined period of time, however, the solder IE! will be raised toits melting point and cause release of the resilient member It. It willbe observed in 2 that a portion of the body member is cut away asindicated by the reference numeral 40 to permit additional movement ofthe resilient member 16, thus insuring that the electrical connection iscompletely broken.

Since the lagging element 25! is of relatively small mass with respectto the stiffness of the resilient member it on which it is carried,there will be no vibration, thereby assuring a fuse of long life. Byutilizing the heat developed in the elements 55 ll, instead of by theuse of a separate heating roll, I have found that a lagging element ofcomparatively small mass will provide the desired time delaycharacteristics.

The fact that the lagging element is spaced from the wall of the bodymember contributes to this result since the spacing causes it to bethermally insulated from the body member since the solder flows betweenthe lagging element and the free end portion of the resilient strips andcoats all of these parts, including the end of the link, the threeseparate elements are all 1 sociated in such heat transmittingrelationship that the temperature of each element with respect to theothers will be the same. Thus localized heating of the solder will beavoided, which contributes to the uniformity of electricalcharacteristics of the fuse.

In the modified form of my invention shown in Figs. 9 to 11 inclusive,the parts which are substantially the same as the parts shown in -Fig. lare indicated by the same reference numerals primed. These figures showa type of construction which has a small heat absorbing capacity andhence which .is more adaptable for fuses of smaller rating than. thefuse shown in Figs. 1 to 8 inclusive. Here the separate lagging elementis omitted and the heat absorbing capacity is provided by the solder 46alone.

As shown in Fig. 9, a resilient member 41 is disposed within the hollowbody member ID and glass top H, the resilient member including a portion42 which is gripped between the screw shell 12' and the glass top H. Thefuse link '53 is disposed more or less vertically, the bottom portionthereof being suitably secured to the center cap I5. passes through aslot 45 formed in the end portion 44 of the resilient member All. Theend portion 44 is cup shaped, as shown in Fig. so that the amount ofsolder 46 which is used in securing the parts together may be readilypredetermined.

In this embodiment, as in the embodiment shown in Figs. 1. to 8inclusive, the heat absorbing material is spaced from the body member ofthe fuse so as to be thermally insulated therefrom. Also, the endportion of the resilient strip is slotted to receive the end of the fuselink, and the parts are secured together in heat transmittingrelationship on the opposite side of the resilient element. Thus, thereis less likelihood The upper end of the fuse link of the parts pullingaway from each other until all of the parts in the vicinity of the endpor tion d lhave been uniformly raised to the temperature of the fusingpoint of the solder.

I claim:

1. A fuse comprising a body member, two strips of metal each beingsecured to said body member so as to provide a free end portion for eachof said strips, one of said strips being fiexurally resilient and beingmaintained in flexurally st essed condition by the other of said strips,the free end portion thereof being apertured to receive the free endportion of the other of said strips, and means securing said free endportions to each. other in heat transmitting relationship, said meansincluding a fusible metal which is adapted to release said resilientstrip under small overload conditions, said second mentioned stripconstituting a fuse link which is adapted to blow under short circuitconditions. and means for connect-=- ing the outer ends of each of saidstrips into a circuit whereby said strips will be in series circuit witheach other.

2. A fuse as claimed in claim 1 in which the apertured free end portionof said flexurally resilient strip is cup shaped, and wherein saidfusible metal is disposed in said cup shaped end portion.

3. A. fuse comprising a body member, two strips of metal each beingsecured to said body member so as to provide a free end portion for eachof said strips, one of said strips being ilexurally resilient and beingmaintained in stressed condition by the other of said strips, the freeend portion thereof being apertured to receive the free end portion ofthe other of said strips, said second mentioned strip constituting afuse link, and means securing said free end portions to each other inheat transmitting relationship, said means including a body of metalfusibly secured to both of said free end portions of said fuse strip anddisposed on the side of said resilient strip which is opposite from theside facing aid fuse link so as to interlock with said resilient strip,said apertured free end portion being shaped so as to form a support forsaid body of metal during assembly.

4. A fuse comprising .abody member, two strips of metal each beingsecured to said body member so as to provide a free end portion for eachof said strips, fusible means securing the free end portions of saidstrips to each other, and ala ing element secured to the free end of atleast one of said strips by said fusible means and being spaced fromsaid body member, one of said strips being flexurally resilient andbeing maintained in stressed oonditionby the other of said strips, theother one of said strips being provided with a portion of reduced crosssection adapted to blow under short circuit conditions and adapted togenerate heat under small overload conditions, said fusible meanscomprising a low melting point solder which connects said laggingelement and said strips together in heat transmitting relationship,whereby said lagging element will receive and absorb the heat generatedby said fuse link in order to maintain theconnected portions of saidstrips and said solder at substantially uniform temperature with respectto each other.

5. A plug fuse comprising a body member having a center contact, aresilient member anchored to said center contact and extending generallyparallel to the longitudinal axis of said body member, said body memberbeing provided with a substantially cylindrical wall portion, a fuselink having one portion disposed along the outer surface of said wallportion and having a second portion extending inwardly from the upperedge of said waH portion toward the free end portion of said resilientmember, said free end portion being slotted to receive the end of saidfuse link, solder connecting the ends of said fuse link and resilientmember whereby said resilient member is maintained in a fiexurallystressed condition, and a lagging element mounted on said resilientstrip at the point where it is engaged by said fuse link.

6. A fuse as claimed in claim in which said fuse link as provided with asubstantially right angle bend at the upper edge of said wall portion,said fuse link being. provided with laterally extending wing portionsengaging the upper edge of said wall portion. and serving to determinethe position of said bend, said fuse link also being provided with alaterally extending tongue engaging said resilient member at a pointadjacent to said slot to determine the extent of penetration of the endof said fuse link into said slot, whereby the spacing between the freeend portion of said resilient member and said wall portion isdetermined.

7. A fuse comprising a flexurally resilient member anchored at one end,a fuse link anchored at one end, fusible means securing the free ends ofsaid resilient member and said fuse link so as to maintain saidilexurally resilient member in a fiexurally stressed condition, meansfor connecting the outer ends of said resilient member and said fuselink into an external circuit, and a lag-= ging element carried by saidresilient member at the point where said resilient member and said fuselink are secured to each other, said lagging element being secured tosaid resilient member by said fusible means, the resilience of saidflexurally resilient member constituting the sole means for causingseparation of said fuse link and said fiexurally resilient member whensaid fusible means becomes softened under overload conditions.

8. A fuse as claimed in claim 7 in which said lagging element is in theform of a metallic slug having a mass which is great with respect to themass of said fusible means.

9. A fuse as claimed in claim 7 having a casing of insulating materialand in which said lagging element is spaced from said casing in order tobe thermally insulated therefrom. 10. A fuse as claimed in claim 7 inwhich said fuse link is provided with a portion of reduced cross sectionwhich is adapted to blow under short circuit conditions.

11. A fuse comprising a body member, two strips of metal each beingsecured to said body member so as to provide a free end portion for eachof said strips, one of said strips being fiexurally resilient and beingmaintained in flexurally stressed condition by the other of said strips,the free end portion thereof being apertured to receive the free endportion of the other of said strips, means securing said free endportions to each other in heat transmitting relationship, said meansincluding a fusible metal which is adapted to release said resilientstrip under small overload conditions, said second mentioned stripconstituting a fuse link which is adapted to blow under short circuitconditions, the apertured free end portion of said flexurally resilientstrip being provided with wings, and said means for securing said freeend portions to each other including a separate lagging element engagedby said wings, said fusible metal being disposed between said laggingelement and said wings and between said lagging element and the free endportion of said fuse link.

12. A fuse comprising a body member, two strips of metal each beingsecured to said body member so as to provide a free end portion for eachof said strips, one of said strips being l'iexurally resilient and beingmaintained in flexurally stressed condition by the other of said strips,the free end portion thereof being apertured to receive the free endportion of the other of said strips, means securing said free endportions to each other in heat transmitting relationship, said meansincluding a fusible metal which is adapted to release said resilientstrip under small overload conditions, said second mentioned stripconstituting a fuse link which is adapted to blow under short circuitconditions, and said resilient strip including a first portion disposedsubstantially parallel to a surface portion of said body member andanchored against angular displacement with respect thereto, and a secondportion disposed between said first portion and said free end portion,said second portion being provided with a bend, the free end portions ofsaid strips being substantially perpendicular to each other whereby themovement of the free end portion of said resilient strip i in adirection substantially parallel to the axis of the free end portion ofsaid fuse link strip.

GUST A. BERTHEL.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,218,274 Wood Oct. 15, 19402,224,111 Wood Dec. 3, 1940 2,275,403 Bussman Mar. 10, 1942 2,286,518Taylor June 16, 1942 2,313,281 Taylor Mar. 9, 1943

